Current slope control method and apparatus for power driver circuit application

ABSTRACT

A low side driver includes a first transistor coupled in series with a second transistor at a low side voltage node for a load. A capacitance is configured to store a voltage and a voltage buffer circuit has an input coupled to receive the voltage stored by the capacitance and an output coupled to drive a control node of the second transistor with the stored voltage. A current source supplies current through a switch to the capacitance and the input of the voltage buffer circuit. The switch is configured to be actuated by an oscillating enable signal so as to cyclically source current from the current source to the capacitance and cause a stepped increase in the stored voltage which is applied by the buffer circuit to the control node of the second transistor.

PRIORITY CLAIM

This application claims priority from Chinese Application for Patent No.201210153152.6 filed May 11, 2012, the disclosure of which is herebyincorporated by reference.

BACKGROUND

1. Technical Field

The present invention relates generally to power driver circuits andtheir operation and in particular to method and apparatus forcontrolling current slope of a control signal for a power driver.

2. Introduction

Reference is made to FIG. 1 which shows a circuit diagram of a powerdriver circuit 100. The circuit 100 includes a high side driver 102 anda low side driver 104. The output of the high side driver 102 is coupledto a high side node 106 of a load 108. The output of the low side driver104 is coupled to a low side node 110 of the load 108. In the exemplaryimplementation of FIG. 1, the load 108 is a display panel of an LCD orAMOLED type and the high side node 106 and low side node 110 are thevoltage supply nodes for the display panel. It will be understood,however, that the load 108 may comprise any suitable load driven fromboth the high and low side.

The high side driver 102 comprises a pair of series connectedtransistors 120 and 122. The transistors 120 and 122 are of then-channel MOSFET type coupled in series through their source-drainpaths. It will be understood that transistors of other type may insteadbe used, the reference to n-channel MOSFET devices being exemplary onlyof a preferred implementation. P-channel MOSFETs, combinations ofn-channel and p-channel MOSFETs, bi-polar devices and/or IGFET typedevices may alternatively be used.

The transistor 120 includes a conduction (drain) terminal coupled to afirst power supply node 124 and a conduction (source) terminal coupledto the high side node 106. A control (gate) terminal of the transistor120 is coupled to a first control node 126. The transistor 122 includesa conduction (drain) terminal coupled to the high side node 106 and aconduction (source) terminal coupled to a second power supply node 128.A control (gate) terminal of the transistor 122 is coupled to a secondcontrol node 130.

The low side driver 104 comprises a pair of series connected transistors140 and 142. The transistors 140 and 142 are of the n-channel MOSFETtype coupled in series through their source-drain paths. It will beunderstood that transistors of other type may instead be used, thereference to n-channel MOSFET devices being exemplary only of apreferred implementation. P-channel MOSFETs, combinations of n-channeland p-channel MOSFETs, bi-polar devices and/or IGFET type devices mayalternatively be used.

The transistor 140 includes a conduction (drain) terminal coupled to athird power supply node 144 and a conduction (source) terminal coupledto the low side node 110. A control (gate) terminal of the transistor140 is coupled to a third control node 146. The transistor 142 includesa conduction (drain) terminal coupled to the low side node 110 and aconduction (source) terminal coupled to a fourth power supply node 148.A control (gate) terminal of the transistor 142 is coupled to a fourthcontrol node 150.

The first and third power supply nodes 124 and 144 are preferablycoupled to receive high supply voltages (for example, Vdd1 and Vdd2).These may, for example, be different high supply voltages, or the samehigh supply voltage, depending on circuit application.

The second and fourth supply nodes 128 and 148 are preferably coupled toreceive low supply voltages. These may, for example, be different lowsupply voltages, or the same low supply voltage (for example, ground),depending on circuit application.

Reference is now made to FIG. 2 which illustrates voltage waveforms forthe voltage signals at the high side node 106 (voltage signal Va) andlow side node 110 (voltage signal Vb). These waveforms are specific tothe exemplary implementation of FIG. 1 where the load 108 is a displaypanel of an LCD or AMOLED type. It will be understood, however, thathigh and low side waveforms having a similar shape and timing may beapplicable with other types of loads.

During a period of time associated with resetting the display panel load108 (of an LCD or AMOLED type), the high side driver 102 and low sidedriver 104 are controlled by application of appropriate controlsignaling to the first, second, third and fourth control nodes 126, 130,146 and 150 of the transistors 120, 122, 140 and 142, respectively, topull down the voltage at the high side node 106 (voltage signal Va) asindicated at reference 160. The reset time period terminates when thevoltage at the high side node 106 (voltage signal Va) returns high.During a first time period t1 associated with initially pulling down thevoltage at the high side node 106, it is important to exercise controlover the downward voltage slope. In particular, there is a need tocontrol the slope in a manner which ensures that no voltage/currentspike is introduced during the power driving operation.

During a period of time associated with emission in the display panelload 108 (of an LCD or AMOLED type), the high side driver 102 and lowside driver 104 are controlled by application of appropriate controlsignaling to the first, second, third and fourth control nodes 126, 130,146 and 150 of the transistors 120, 122, 140 and 142, respectively, topull down the voltage at the low side node 110 (voltage signal Vb) asindicated at reference 162. The emission time period terminates when thevoltage at the low side node 110 (voltage signal Vb) returns high.During a second time period t2 associated with initially pulling downthe voltage at the low side node 110, it is important to exercisecontrol over the downward voltage slope. In particular, there is a needto control the slope in a manner which ensures that no voltage/currentspike is introduced during the power driving operation.

SUMMARY

In an embodiment, a circuit comprises: a low side driver including afirst transistor coupled in series with a second transistor at a lowside voltage node, said low side voltage node configured to be coupledto a load; a capacitance configured to store a voltage; a voltage buffercircuit having an input coupled to receive the voltage stored by thecapacitance and an output coupled to drive a control node of the secondtransistor with said stored voltage; a first current source; and a firstswitch coupled between the first current source and the input of thevoltage buffer circuit, wherein said first switch is configured to beactuated by an oscillating enable signal so as to cyclically sourcecurrent from said first current source to said capacitance and cause astepped increase in the stored voltage.

In an embodiment, a circuit that is configured to drive a low sidedriver including a drive transistor coupled between a low side voltagenode of a load and a reference voltage comprises: a first circuitconfigured to sense a threshold voltage of the drive transistor; acapacitance configured to store a voltage; a second circuit configuredto cause the sensed threshold voltage to be stored as an initial voltagestored by said capacitance; a first current source; a first switchcoupled between the first current source and the capacitance, where saidfirst switch configured to be actuated by an oscillating enable signalso as to cyclically source current from said first current source tosaid capacitance and cause a stepped increase of the voltage stored bysaid capacitance from said initial voltage; and a voltage buffer circuithaving an input coupled to receive the voltage stored by the capacitanceand an output coupled to drive a control node of the drive transistorwith said stored voltage.

In another embodiment, a circuit that is configured to drive a low sidedriver including a first drive transistor coupled between a low sidevoltage node of a load and a reference voltage comprises: a controltransistor coupled between the control node of the first drivetransistor and a reference voltage, said control transistor having acontrol node configured to receive a drive control signal; a capacitanceconfigured to store a voltage; a first circuit configured to pre-chargethe control node of the first drive transistor to a voltage which isstored as an initial voltage stored by said capacitance, said firstcircuit actuated in response to said drive control signal; a firstcurrent source; a first switch coupled between the first current sourceand the capacitance, where said first switch configured to be actuatedby an oscillating enable signal so as to cyclically source current fromsaid first current source to said capacitance and cause a steppedincrease in the stored voltage; and a voltage buffer circuit having aninput coupled to receive the voltage stored by the capacitance and anoutput coupled to drive a control node of the first drive transistorwith said stored voltage.

The foregoing and other features and advantages of the presentdisclosure will become further apparent from the following detaileddescription of the embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the disclosure, rather than limiting the scope of theinvention as defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingfigures not drawn to scale, in which like reference numbers indicatesimilar parts, and in which:

FIG. 1 is a circuit diagram of a power driver circuit;

FIG. 2 illustrates voltage waveforms for the voltage signals at the highside node and low side node of the circuit in FIG. 1;

FIG. 3 is a diagram of a first embodiment of an open loop controlcircuit;

FIG. 4 is a diagram of a second embodiment of an open loop controlcircuit;

FIG. 5 is timing and voltage plot illustrating operation of the circuitof FIG. 3; and

FIG. 6 is a timing and voltage plot illustrating operation of thecircuit of

FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is now made to FIG. 3 showing a diagram of a first embodimentof an open loop control circuit 200. The circuit 200 includes a highside driver 202 and a low side driver 204. The output of the high sidedriver 202 is coupled to a high side node (ELVDD) 206 of a load 208. Theoutput of the low side driver 204 is coupled to a low side node 210(ELVSS) of the load 208. In the exemplary implementation of FIG. 3, theload 208 is a display panel of an LCD or AMOLED type which includes aplurality of diodes 270, each diode coupled in series with a controltransistor 272 between the high side node (ELVDD) 206 and the low sidenode 210 (ELVSS). It will be understood, however, that the load 208 maycomprise any suitable load driven from both the high and low side.

The high side driver 202 comprises a pair of series connectedtransistors 220 and 222. The transistor 220 is of the p-channel MOSFETtype and the transistor 222 is of the re-channel MOSFET type. Thesetransistors are coupled in series through their source-drain paths. Itwill be understood that transistors of other type may instead be used,the reference to p-channel and n-channel MOSFET devices being exemplaryonly of a preferred implementation. P-channel MOSFETs, n-channelMOSFETs, bi-polar devices and/or IGFET type devices may alternatively beused.

The transistor 220 includes a conduction (source) terminal coupled to afirst power supply node 224 and a conduction (drain) terminal coupled tothe high side node 206. A control (gate) terminal of the transistor 220is coupled to a first control node 226 (PG1). The transistor 222includes a conduction (drain) terminal coupled to the high side node 206and a conduction (source) terminal coupled to a second power supply node228. A control (gate) terminal of the transistor 222 is coupled to asecond control node 230 (NG1).

The low side driver 204 comprises a pair of series connected transistors240 and 242. The transistor 240 is of the p-channel MOSFET type and thetransistor 242 is of the re-channel MOSFET type. These transistors arecoupled in series through their source-drain paths. It will beunderstood that transistors of other type may instead be used, thereference to p-channel and n-channel MOSFET devices being exemplary onlyof a preferred implementation. P-channel MOSFETs, n-channel MOSFETs,bi-polar devices and/or IGFET type devices may alternatively be used.

The transistor 240 includes a conduction (source) terminal coupled to athird power supply node 244 and a conduction (drain) terminal coupled tothe low side node 210. A control (gate) terminal of the transistor 240is coupled to a third control node 246 (PG2). The transistor 242includes a conduction (drain) terminal coupled to the low side node 210and a conduction (source) terminal coupled to a fourth power supply node248. A control (gate) terminal of the transistor 242 is coupled to afourth control node 250 (NG2).

The first and third power supply nodes 224 and 244 are preferablycoupled to receive high supply voltages. These may, for example, bedifferent high supply voltages, or the same high supply voltage,depending on circuit application.

The second and fourth supply nodes 228 and 248 are preferably coupled toreceive low supply voltages. These may, for example, be different lowsupply voltages, or the same low supply voltage (for example, ground),depending on circuit application.

The circuit 200 further comprises a first switch 260 coupled between thelow side node 210 and a first intermediate node 262, and a second switch264 coupled between the first intermediate node 262 and the fourthcontrol node 250 (NG2). The first and second switches 260 and 264 maycomprise transistor switches as well known to those skilled in the art.The state of the first and second switches 260 and 264 is commonlycontrolled by a first enable signal (EN_VTH). When the first enablesignal is inactive (logic low, for example), the first and secondswitches 260 and 264 are open. However, when the first enable signal isactive (logic high, for example), the first and second switches 260 and264 are closed, and this causes the shunting of the conduction (drain)terminal (at the low side node 210) to the fourth control node 250 (NG2)for the transistor 242. A first current source 266 is coupled to sourcefixed current into the first intermediate node 262. Thus, when the firstenable signal is active and the first and second switches 260 and 264are closed, and a fixed current is sourced to charge the parasitic gatecapacitance at the fourth control node 250 (NG2) of transistor 242.

The circuit 200 further comprises an analog-to-digital converter (ADC)circuit 268 having an input coupled through a third switch 274 to thefourth control node 250 (NG2). The third switch 274 may comprise atransistor switch as well known to those skilled in the art. The stateof the third switch 274 is commonly controlled with the states of thefirst and second switches 260 and 264 by the first enable signal(EN_VTH). Thus, when the first enable signal is inactive (logic low, forexample), the first, second and third switches 260, 264 and 274 areopen. However, when the first enable signal is active (logic high, forexample), the first, second and third switches 260, 264 and 274 areclosed. In this configuration the input of the analog-to-digitalconverter circuit 268 is coupled to the fourth control node 250 (NG2),and the analog-to-digital converter circuit 268 operates to sample thevoltage at the fourth control node 250 (NG2), when the conduction(drain) terminal is shunted to the low side node 210 to the fourthcontrol node 250 (NG2), and thus measure the threshold voltage of thetransistor 242. The sampled threshold voltage is stored in a latchcircuit 276 coupled to the output of the analog-to-digital convertercircuit 268.

The circuit 200 still further comprises digital-to-analog converter(DAC) circuit 280 having a first input coupled to an output of the latchcircuit 276. The digital-to-analog converter circuit 280 functions toconvert the digital value of the measured threshold voltage of thetransistor 242 (as stored in the latch) to an analog output value. Anadjustment circuit in the form of a negative temperature coefficientresistance 282 is coupled to a second input of the digital-to-analogconverter circuit 280. This negative temperature coefficient resistance282 functions as a temperature sensor operable to sense a temperature ofthe transistor 242. The sensed temperature of the transistor 242 is usedto offset the digital value of the measured threshold voltage of thetransistor 242 by a value and thus compensate for variation intransistor threshold voltage as a function of temperature. The use ofthe temperature adjustment circuit may not be needed in all applicationsof the circuit 200, and thus presents an optional feature. Those skilledin the art will recognize situations where use of the temperatureadjustment circuit is advantageous, and further will be able toconfigure the adjustment circuit and digital-to-analog converter circuit280 to implement an accurate offsetting of the digital value of themeasured threshold voltage of the transistor 242 to compensate forvariation in transistor threshold voltage as a function of temperature.

The circuit 200 further comprises a fourth switch 284 coupled between anoutput of the digital-to-analog converter circuit 280 and a secondintermediate node 286. A capacitor 288 is coupled between the secondintermediate node 286 and a reference voltage node (in this casecomprising the second and fourth supply nodes 228 and 248 receiving thelow supply voltage (ground)). The capacitor 288 stores a voltage(VREF_BUF) applied to the non-inverting input of a unitary gain voltagebuffer circuit 290. The circuit 290 in one implementation comprises anoperational amplifier circuit having its non-inverting input terminalcoupled to the second intermediate node 286, and its inverting inputterminal coupled to its output terminal. The output terminal of thebuffer circuit 290 is coupled through a fifth switch 292 to the fourthcontrol node 250 (NG2) for transistor 242. The fourth and fifth switches284 and 292 may comprise transistor switches as well known to thoseskilled in the art. The state of the fourth switch 284 is controlled bya second enable signal (ENA). The state of the fifth switch 292 iscontrolled by a third enable signal (ENB). When the second and thirdenables signals are inactive (logic low, for example), the fourth andfifth switches 284 and 292 are open. However, when the second and thirdenable signals are active (logic high, for example), the fourth andfifth switches 284 and 292 are closed.

The circuit 200 still further comprises a second current source 296coupled to source fixed current into the second intermediate node 286through a sixth switch 298. The sixth switch 298 may comprise atransistor switch as well known to those skilled in the art. The stateof the sixth switch 298 is controlled by a fourth enable signal(EN_OSC). When the fourth enable signal is inactive (logic low, forexample), the sixth switch 298 is open. However, when the fourth enablesignal is active (logic high, for example), the sixth switch 298 isclosed, and a fixed current is sourced to charge the capacitor 288 andincrease the voltage (VREF_BUF) applied to the non-inverting input of aunitary gain voltage buffer circuit 290. The fourth enable signal is anoscillating signal, and thus cyclic activations of the sixth switch 298will produce a stepped increase in the voltage (VREF_BUF). The value ofthe fixed current output by the second current source 296 is set by thevalue of a control resistor 300. The fourth enable signal (EN_OSC) may,for example, be generated by logically ANDing an enable signal with anoscillating clock signal, wherein the oscillating clock signal maycomprise an oscillating clock signal operable to clock operationsperformed in connection with controlling and driving the load 208. Itwill be understood, however, that in some implementations an oscillatingsignal is not needed, and as such the switch 298 may be controlled bythe enable signal directly.

Operation of the circuit 200 in performing an open loop control functionis as follows:

STEP 1: Measurement of the threshold voltage of the transistor 242. Asdiscussed above, the first enable signal (EN_VTH) is controlled to beactive, and the first, second and third switches 260, 264 and 274 areclosed (FIG. 5, reference 350). This shorts the drain and gate(conduction and control) terminals of the transistor 242 together. Thefirst current source 266 charges the gate (control) terminal of thetransistor 242 based on a fixed sourced current value (for example, 100uA), and the analog-to-digital converter circuit 268 samples the gate(control) node voltage to thus measure the threshold voltage of thetransistor 242 (FIG. 5, reference 352). The sampled threshold voltage isstored in the latch circuit 276. That sampled threshold voltage,adjusted to account for temperature (if needed or desired), is madeavailable as a voltage value present at the output of thedigital-to-analog converter circuit 280. In a preferred implementation,this threshold voltage measurement operation is performed at powering upof the load 208 by supply voltage Vin. The power on reset (POR) signalschange of state of the first enable signal (EN_VTH) and closure of thefirst, second and third switches 260, 264 and 274 to make and store thethreshold measurement. Alternatively, the measurement can be made duringoperation when there is a suitable free time interval according to theapplication. In this case, the negative temperature offset compensationis not required as the measurement of threshold voltage is being made inreal time (and at a current operating temperature).

STEP 2: Pre-charge the gate (control) node of transistor 242 to thesampled threshold voltage. Next, the first enable signal (EN_VTH) iscontrolled to be inactive, and the second and third enable signals (ENAand ENB) are controlled to be active. This turns on the fourth and fifthswitches 284 and 292. The voltage (VREF_BUF) stored on the capacitor 288is initially set to equal the voltage value present at the output of thedigital-to-analog converter circuit 280 (FIG. 5, reference 354). Thebuffer circuit 290 passes that voltage (VREF_BUF) for application to thegate (control) node of the transistor 242. It is important to recognizethat the buffer circuit 290 must be designed with a large currentcapability in order to quickly charge the gate capacitance of thetransistor 242 to the sampled threshold voltage (possibly withtemperature compensation).

STEP 3: Use adjustable current to charge the gate (control) node oftransistor 242 so as to control the slope of transistor 242 turn on.Next, the second enable signal (ENA) is deactivated, the third enablesignal (ENB) remains active, and the fourth enable signal (EN_OSC) iscontrolled to be active. As discussed above, the fourth enable signal(EN_OSC) is an oscillating signal, and thus current from the secondcurrent source 296 is cyclically sourced to the capacitor 288. Thus,current is injected with each oscillating pulse causing a correspondingmulti-step increase in capacitor voltage (FIG. 5, reference 356).Because the second enable signal is active, the increased capacitorvoltage (VREF_BUF) is passed for application to the gate (control) nodeof the transistor 242. As the gate voltage increases, the transistor 242correspondingly turns on. The slope of the transistor 242 turn on iscontrolled by the rate of change in the capacitor voltage (VREF_BUF) andthus is a function of the fixed current of the second current source 296and the duty cycle of the oscillating fourth enable signal (EN_OSC).

Reference is now made to FIG. 4 showing a diagram of a second embodimentof an open loop control circuit 500. The circuit 500 includes a highside driver 502 and a low side driver 504. The output of the high sidedriver 502 is coupled to a high side node (ELVDD) 506 of a load 508. Theoutput of the low side driver 504 is coupled to a low side node 510(ELVSS) of the load 508. In the exemplary implementation of FIG. 4, theload 508 is a display panel of an LCD or AMOLED type which includes aplurality of diodes 270, each diode coupled in series with a controltransistor 272 between the high side node (ELVDD) 506 and the low sidenode 510 (ELVSS). It will be understood, however, that the load 508 maycomprise any suitable load driven from both the high and low side.

The high side driver 502 comprises a pair of series connectedtransistors 520 and 522. The transistor 520 is of the p-channel MOSFETtype and the transistor 522 is of the re-channel MOSFET type. Thesetransistors are coupled in series through their source-drain paths. Itwill be understood that transistors of other type may instead be used,the reference to p-channel and n-channel MOSFET devices being exemplaryonly of a preferred implementation. P-channel MOSFETs, n-channelMOSFETs, bi-polar devices and/or IGFET type devices may alternatively beused.

The transistor 520 includes a conduction (source) terminal coupled to afirst power supply node 524 and a conduction (drain) terminal coupled tothe high side node 506. A control (gate) terminal of the transistor 520is coupled to a first control node 526 (PG1). The transistor 522includes a conduction (drain) terminal coupled to the high side node 506and a conduction (source) terminal coupled to a second power supply node528. A control (gate) terminal of the transistor 522 is coupled to asecond control node 530 (NG1).

The low side driver 504 comprises a pair of series connected transistors540 and 542. The transistor 540 is of the p-channel MOSFET type and thetransistor 542 is of the re-channel MOSFET type. These transistors arecoupled in series through their source-drain paths. It will beunderstood that transistors of other type may instead be used, thereference to p-channel and n-channel MOSFET devices being exemplary onlyof a preferred implementation. P-channel MOSFETs, n-channel MOSFETs,bi-polar devices and/or IGFET type devices may alternatively be used.

The transistor 540 includes a conduction (source) terminal coupled to athird power supply node 544 and a conduction (drain) terminal coupled tothe low side node 510. A control (gate) terminal of the transistor 540is coupled to a third control node 546 (PG2). The transistor 542includes a conduction (drain) terminal coupled to the low side node 510and a conduction (source) terminal coupled to a fourth power supply node548. A control (gate) terminal of the transistor 542 is coupled to afourth control node 550 (NG2).

The first and third power supply nodes 524 and 544 are preferablycoupled to receive high supply voltages. These may, for example, bedifferent high supply voltages, or the same high supply voltage,depending on circuit application.

The second and fourth supply nodes 528 and 548 are preferably coupled toreceive low supply voltages. These may, for example, be different lowsupply voltages, or the same low supply voltage (for example, ground),depending on circuit application.

The circuit 500 comprises a comparator circuit 551 having a first(positive) input terminal coupled to the low side node 510 and a second(negative) input terminal coupled, through a voltage offset 552 (forexample, of 2.0V), to the conduction (source) terminal of the transistor540 (i.e., coupled to the third power supply node 544). The comparatorcircuit 551 has an output from which is generated a first enable (EN)signal. The comparator circuit 551 functions as a sensing circuit todetect when the transistor 542 has been turned on. The first enablesignal is inactive (for example, logic low) when the transistor 542sensed to be off, and is active (for example, logic high) when thetransistor is sensed to be on (said sensing triggered by the voltage atthe low side node 510 differing from the voltage at the third powersupply node 544 by more than the voltage offset 552).

An activation transistor 556 is coupled between the fourth control node550 (NG2) and fourth supply node 548. Specifically, the activationtransistor 556 has a conduction (drain) terminal coupled to the fourthcontrol node 550 (NG2) and a conduction (source) terminal coupled to thefourth supply node 548. In this configuration, the transistor 556 is ann-channel MOSFET type transistor. The control (gate) terminal oftransistor 556 receives an activation signal (NDRIVER(bar)). When theactivation signal (NDRIVER(bar)) is logic high, the transistor 556 isturned on and the fourth control node 550 (NG2) is clamped to the fourthsupply node 548 (ground). This prevents turn on of the transistor 542.Conversely, when the activation signal (NDRIVER(bar)) is logic low, thetransistor 556 is turned off and this permits transistor 542 to beturned on.

The first enable (EN) signal is logically combined with the activationsignal (NDRIVER; note: this is the logical inversion of the signalpreviously discussed) in logic circuitry 560 to generate a second enable(PRE_CHG) signal. The logic circuitry 560 comprises a NOT gate 562 whichinverts the logic state of the first enable (EN) signal, and an AND gate564 which logically combines the inverted first enable (EN) signal withthe activation signal (NDRIVER). The logic circuitry 560 essentiallyfunctions as a pulse generator that outputs a one-shot pulse for thesecond enable (PRE CHG) signal. This one-shot pulse has a leading edgeresponsive to the change in state of the activation signal (NDRIVER) tologic high (i.e., the logic low activation signal NDRIVER(bar)), and atrailing edge responsive to the logic high state of the first enable(EN) signal (i.e., the sensing of the transistor 542 turning on with acertain voltage drop in the low side node 510 (ELVSS) voltage asdiscussed above).

The circuit 500 further comprises a first switch 580 coupled between thefourth control node 550 (NG2) and a first intermediate node 582, and asecond switch 584 coupled between the first intermediate node 582 and asecond intermediate node 590. The first and second switches 580 and 584may comprise transistor switches as well known to those skilled in theart. The state of the first and second switches 580 and 584 is commonlycontrolled by the second enable (PRE_CHG) signal. When the second enablesignal is inactive (logic low, for example), the first and secondswitches 580 and 584 are open. However, when the second enable signal isactive (logic high, for example), the first and second switches 580 and584 are closed. A first current source 596 is coupled to source fixedcurrent into the first intermediate node 582.

In response to a logic low activation signal (NDRIVER(bar)), the logicalcomplement activation signal (NDRIVER) goes to logic high and the secondenable (PRE_CHG) signal is active. The first and second switches 580 and584 are closed. Current from the first current source 596 charges theparasitic gate capacitance at the fourth control node 550 (NG2) of thetransistor 542 and the fourth control node 550 (NG2) voltage rises. Asthe transistor 542 begins to turn on, current flows from the low sidenode 510 to the fourth supply node 548 (ground), and the voltage on thelow side node 510 (ELVSS) decreases. This decrease in low side node 510(ELVSS) voltage, indicative of transistor 542 being turned on, is sensedby the comparator circuit 551, whose output first enable (EN) signalswitches from the inactive (for example, logic low) state to the active(for example, logic high) state. The second enable (PRE_CHG) signal isthen switched to an inactive logic state and the first and secondswitches 580 and 584 are opened.

A capacitor 598 is coupled between a second intermediate node 590 and areference voltage node (in this case comprising the second and fourthsupply nodes 528 and 548 receiving the low supply voltage (ground)). Thecapacitor 598 stores a voltage (VREF_BUF) applied to the non-invertinginput of a unitary gain voltage buffer circuit 600. The circuit 600 inone implementation comprises an operational amplifier circuit having itsnon-inverting input terminal coupled to the second intermediate node590, and its inverting input terminal coupled to its output terminal.The output terminal of the circuit 600 is coupled to the fourth controlnode 550 (NG2).

Thus, when the second enable (PRE_CHG) signal is active and the firstand second switches 580 and 584 are momentarily closed by the one-shotpulse of the second enable signal, a fixed current is sourced to chargethe parasitic gate capacitance of the fourth control node 550 (NG2) forthe transistor 242 to a threshold voltage of the transistor 542. Thatcharged threshold voltage is further stored through charge-sharing inthe capacitor 598 at the second intermediate node 590 as an initialvalue of the voltage (VREF_BUF). This charging operation lasts until thesensed voltage drop of low side node 510 (ELVSS) voltage exceeds thethreshold set by the voltage offset 552 and the first and secondswitches 580 and 584 are opened.

The circuit 500 still further comprises a second current source 616coupled to source fixed current into the second intermediate node 590through a third switch 618. The third switch 618 may comprise atransistor switch as well known to those skilled in the art. The stateof the third switch 618 is controlled by a third enable signal (EN_OSC).When the third enable signal is inactive (logic low, for example), thethird switch 618 is open. However, when the third enable signal isactive (logic high, for example), the third switch 618 is closed, and afixed current is sourced to charge the capacitor 598 and increase thevoltage (VREF_BUF) applied to the non-inverting input of the unitarygain voltage buffer circuit 600. The third enable signal is anoscillating signal, and thus cyclic activations will produce a steppedincrease in the voltage (VREF_BUF). The value of the fixed currentoutput by the second current source 616 is set by the value of a controlresistor 620. The third enable signal (EN_OSC) is generated by a logiccircuit which includes an AND gate 622 operable to logically AND thefirst enable (EN) signal with a oscillating clock (OSC) signal, whereinthe oscillating clock signal may comprise an oscillating clock signaloperable to clock operations performed in connection with controllingand driving the load 508. The comparator circuit 551 changes the logicstate of the first enable (EN) signal to logic high in response tosensing that transistor 542 has turned on. This enables application ofthe oscillating third enable signal (EN_OSC) to actuate the third switch618.

Operation of the circuit 500 in performing an open loop control functionis as follows:

STEP 1: Pre-charge phase. Transistor 542 is controlled through itsfourth control node 550 (NG2) in response to the activation signal(NDRIVER(bar)) transitioning from logic high to logic low (and thusturning off transistor 556), thus permitting a turn on of transistor 542(FIG. 6, reference 650). The complement of the activation signal(NDRIVER) causes a state change in the second enable (PRE_CHG) signal tologic high and the first and second switches 580 and 584 are closed(FIG. 6, reference 652). The positive input and output of the unitarygain voltage buffer circuit 600 are shorted together so that thepositive input terminal voltage follows the voltage at the fourthcontrol node 550 (NG2), and the first current source 596 charges thegate (control) terminal (parasitic capacitance) of the transistor 542based on a fixed sourced current value (for example, 100 uA). Thiscauses the transistor 542 to begin to turn on (FIG. 6, reference 654).With this turn on, the low side node 510 (ELVSS) voltage drops. Thisdrop in voltage sensed by the comparator circuit 551, and the logicaloutput of the comparator circuit which generates the first enable (EN)signal changes state (FIG. 6, reference 656). The change in state of thefirst enable (EN) signal is processed by NOT gate 562 and AND gate 564to terminate the one-shot pulse of the second enable (PRE_CHG) signal.This precharge operation is helpful to save time required for turning onthe transistor 542.

STEP 2: Use adjustable current to charge the gate (control) node oftransistor 542 so as to control the slope of transistor 542 turn on.Following completion of the pre-charge, the first and second switches580 and 584 open (with the change in state of the first enable (EN)signal). The active second enable signal (EN) controls application ofthe oscillating the third enable signal (EN_OSC) to cyclically turn onthe third switch 618 and thus current is injected with each oscillatingpulse causing a corresponding multi-step increase in capacitor voltage(FIG. 6, reference 658). The increased capacitor voltage (VREF_BUF) ispassed for application to the gate (control) node of the transistor 542.As the gate voltage increases, the transistor 542 correspondingly turnson. The slope of the transistor 542 turn on is controlled by the rate ofchange in the capacitor voltage (VREF_BUF) and thus is a function of thefixed current of the second current source 616 and the duty cycle of theoscillating third enable signal (EN_OSC).

What is claimed is:
 1. A circuit, comprising: a low side driverincluding a first transistor coupled in series with a second transistorat a low side voltage node, said low side voltage node configured to becoupled to a load; a capacitance configured to store a voltage; avoltage buffer circuit having an input coupled to receive the voltagestored by the capacitance and an output coupled to drive a control nodeof the second transistor with said stored voltage; a first currentsource; and a first switch coupled between the first current source andthe input of the voltage buffer circuit, wherein said first switch isconfigured to be actuated by an enable signal to source current fromsaid first current source to said capacitance and cause a steppedincrease in the stored voltage.
 2. The circuit of claim 1, wherein theenable signal is an oscillating enable signal so as to cyclically sourcecurrent from said first current source to said capacitance and cause amulti-step increase in the stored voltage.
 3. The circuit of claim 1,further comprising a circuit configured to sense a threshold voltage ofthe second transistor and store said sensed threshold voltage as aninitial value of the voltage stored in said capacitance.
 4. The circuitof claim 3, wherein the circuit configured to sense the thresholdvoltage of the second transistor comprises: a voltage sensing circuit;and a second switch coupled between the control node of the secondtransistor and an input of the voltage sensing circuit and selectivelyactuated to permit the voltage sensing circuit to sense a voltage at thecontrol node of the second transistor, said sensed voltage beingindicative of the threshold voltage of the second transistor.
 5. Thecircuit of claim 4, wherein the circuit configured to sense thethreshold voltage of the second transistor further comprises: a thirdswitch coupled between a conduction node of the second transistor and anintermediate node; a fourth switch coupled between the intermediate nodeand control node of the second transistor; wherein the second, third andfourth switches are selectively actuated in common when sensing thevoltage at the control node of the second transistor during a voltagesensing operation.
 6. The circuit of claim 5, wherein the circuitconfigured to sense the threshold voltage of the second transistorfurther comprises a second current source configured to source currentto said intermediate node.
 7. The circuit of claim 4, wherein thevoltage sensing circuit comprises an analog-to-digital converter andwherein the circuit configured to sense the threshold voltage of thesecond transistor further comprises a latch circuit configured to storea digital value of the sensed voltage at the control node of the secondtransistor.
 8. The circuit of claim 7, wherein the circuit configured tosense the threshold voltage of the second transistor further comprises adigital-to-analog converter configured to convert the digital value toan analog voltage value.
 9. The circuit of claim 8, further comprising atemperature sensing circuit configured to sense a temperature of thesecond transistor, said digital-to-analog converter operable in responseto said sensed temperature to adjust the threshold voltage of the secondtransistor as a function of the sensed temperature.
 10. The circuit ofclaim 8, further comprising a fifth switch coupled between an output ofthe digital-to-analog converter and the input of the voltage buffercircuit, the fifth switch selectively actuated to pass the analogvoltage value for storage as the initial value stored in saidcapacitance.
 11. The circuit of claim 10, further comprising a sixthswitch coupled between the output of the buffer circuit and the controlnode of the second transistor; wherein the fifth and sixth switches areselectively actuated in common when pre-charging the control node of thesecond transistor to the sensed threshold voltage of the secondtransistor.
 12. The circuit of claim 11, wherein the fifth switch isde-actuated and the sixth switch is actuated during the stepped increasein the stored voltage.
 13. The circuit of claim 1, further comprising: asecond switch coupled between the output of the voltage buffer and anintermediate node; a third switch coupled between the intermediate nodeand the input of the voltage buffer; wherein the second and thirdswitches are selectively actuated in common when pre-charging thecontrol node of the second transistor to a pre-charge voltage, saidpre-charge voltage stored as an initial value of the voltage stored insaid capacitance.
 14. The circuit of claim 13, further comprising acontrol transistor coupled between the control node of the secondtransistor and a reference voltage, said control transistor having acontrol node configured to receive a drive control signal, said secondand third switches being selectively actuated in common in response tosaid drive control signal having a logic state which turns off saidcontrol transistor.
 15. The circuit of claim 14, further comprising aturn-on sense circuit configured to sense turn-on of said secondtransistor.
 16. The circuit of claim 15, further comprising logiccircuitry configured to selectively de-actuate the second and thirdswitches in common in response to said turn-on sense circuit sensing ofsecond transistor turn-on.
 17. The circuit of claim 16, wherein saidturn-on sense circuit comprises a comparator circuit configured tocompare a first voltage at the low side voltage node to a referencevoltage.
 18. The circuit of claim 17, wherein said logic circuitrycomprises: a NOT gate configured to invert a signal output from saidcomparator circuit; and an AND gate configured to logically AND anoutput of the NOT gate with said drive control signal.
 19. The circuitof claim 15, further comprising circuitry configured to apply saidoscillating enable signal to said first switch in response to saidturn-on sense circuit sensing turn-on of said second transistor.
 20. Thecircuit of claim 1, wherein said load comprises a display panel of theLCD or AMOLED type.
 21. The circuit of claim 1, further comprising ahigh side driver including a third transistor coupled in series with afourth transistor at a high side voltage node, said high side voltagenode configured to be coupled to said load.
 22. A circuit configured todrive a low side driver including a drive transistor coupled between alow side voltage node of a load and a reference voltage, comprising: afirst circuit configured to sense a threshold voltage of the drivetransistor; a capacitance configured to store a voltage; a secondcircuit configured to cause the sensed threshold voltage to be stored asan initial voltage stored by said capacitance; a first current source; afirst switch coupled between the first current source and thecapacitance, where said first switch configured to be actuated by anenable signal so as to source current from said first current source tosaid capacitance and cause a stepped increase of the voltage stored bysaid capacitance from said initial voltage; and a voltage buffer circuithaving an input coupled to receive the voltage stored by the capacitanceand an output coupled to drive a control node of the drive transistorwith said stored voltage.
 23. The circuit of claim 22, wherein theenable signal is an oscillating enable signal so as to cyclically sourcecurrent from said first current source to said capacitance and cause amulti-step increase in the stored voltage.
 24. The circuit of claim 22,wherein the first circuit configured to sense the threshold voltage ofthe drive transistor comprises: a voltage sensing circuit; a secondswitch coupled between the control node of the drive transistor and aninput of the voltage sensing circuit; a third switch coupled between aconduction node of the drive transistor and an intermediate node; afourth switch coupled between the intermediate node and control node ofthe drive transistor; and a second current source configured to sourcecurrent to said intermediate node; wherein the second, third and fourthswitches are selectively actuated in common when sensing the voltage atthe control node of the second transistor during a voltage sensingoperation.
 25. The circuit of claim 24, wherein the voltage sensingcircuit further comprises: a digital-to-analog converter configured toconvert a digital value of the sensed threshold voltage to an analogvoltage value, said analog voltage value selectively coupled to saidcapacitance for storage as said initial voltage.
 26. A circuitconfigured to drive a low side driver including a first drive transistorcoupled between a low side voltage node of a load and a referencevoltage, comprising: a control transistor coupled between the controlnode of the first drive transistor and a reference voltage, said controltransistor having a control node configured to receive a drive controlsignal; a capacitance configured to store a voltage; a first circuitconfigured to pre-charge the control node of the first drive transistorto a voltage which is stored as an initial voltage stored by saidcapacitance, said first circuit actuated in response to said drivecontrol signal; a first current source; a first switch coupled betweenthe first current source and the capacitance, where said first switchconfigured to be actuated by an enable signal so as to source currentfrom said first current source to said capacitance and cause a steppedincrease in the stored voltage; and a voltage buffer circuit having aninput coupled to receive the voltage stored by the capacitance and anoutput coupled to drive a control node of the first drive transistorwith said stored voltage.
 27. The circuit of claim 26, wherein theenable signal is an oscillating enable signal so as to cyclically sourcecurrent from said first current source to said capacitance and cause amulti-step increase in the stored voltage.
 28. The circuit of claim 26,further comprising: a second switch coupled between the output of thevoltage buffer and an intermediate node; a third switch coupled betweenthe intermediate node and the input of the voltage buffer; a secondcurrent source configured to source current to said intermediate node;wherein the second and third switches are selectively actuated in commonin response to said drive control signal when pre-charging the controlnode of the first drive transistor.
 29. The circuit of claim 28, whereinsaid low side driver further includes a second drive transistor coupledin series with the first drive transistor at the low side voltage node,further comprising a turn-on sense circuit coupled across the seconddrive transistor and configured to sense turn-on of said first drivetransistor and in response thereto selectively de-actuate the second andthird switches in common.
 30. The circuit of claim 29, furthercomprising second circuitry configured to apply said oscillating enablesignal to said first switch in response to said turn-on sense circuitsensing turn-on of said first drive transistor.